1. Field of the Invention
The present invention relates to a solid-state imaging device that utilizes charge-coupled devices (CCDs) that is used as an area sensor, a line sensor, or the like.
2. Description of the Prior Art
Signal charge sensing methods employed for a signal charge sensor that forms the output part of a solid-state imaging device constructed from CCDs include, for example, a current output method, a floating-diffusion amplifier method, and a floating-gate amplifier method. The characteristics such as the sensitivity and dynamic range differ among these signal charge sensing methods, each method having its own advantages and disadvantages. Even in the case of signal charge sensors employing the same sensing method, the characteristics such as the sensitivity and dynamic range vary from one type of sensor to another depending on how the sensors are configured.
FIG. 3 is a diagram showing the structure of a signal charge sensor, with its adjacent circuitry of a solid-state imaging device in which a floatingdiffusion amplifier is used for signal charge sensing. A p-type substrate 31 and an n.sup.+ layer 32 form a floating-diffusion diode FD, while an n.sup.+ layer 33 forms a reset drain RD. A line RS indicates a reset gate. The floating-diffusion diode FD is connected to the gate of an amplifier transistor TR31 by a connecting line 34. The floating-diffusion diode FD, the connecting line 34, the transistor TR31, the reset drain RD, and the reset gate RS constitute the signal charge sensor.
The floating-diffusion diode FD senses the signal charge reaching the final stage (end portion) of a horizontal CCD formed below an output electrode OG and produces an output voltage V.sub.out to deliver to the connecting line 34. The floating-diffusion diode FD has a floating capacitance C1 between the n.sup.+ layer 32 and the output electrode OG, a floating capacitance C2 between the n.sup.+ layer 32 and the reset gate RS, and a capacitance Cd between the n.sup.+ layer 32 and the p-type substrate 31. The line 34 has a floating capacitance Cg. Therefore, when the values of the floating capacitances C1, C2, and Cg are denoted as c1, c2, and cg, respectively, and the value of the capacitance Cd as cd, the capacitance cfd of a capacitor CFD formed in the signal charge sensor having the floating-diffusion diode FD and the connecting line 34 can be given by EQU cfd=c1+c2+cg+cd Equation (1)
On the other hand, the output voltage V.sub.out of the signal charge sensor can be given by EQU V.sub.out =Q.sub.sig /cfd Equation (2)
where Q.sub.sig is the amount of signal charge sensed by the signal charge sensor.
Therefore, when the capacitance cfd of the capacitor CFD in the signal charge sensor is made variable, the output voltage V.sub.out of the signal charge sensor can be made variable. For example, when a variable capacitance diode is connected in parallel to the signal charge sensor, the signal charge sensor can be made to provide a variable output voltage. In this case, however, the capacitance of the signal charge sensor increases by the variable capacitance diode connected in parallel to it, and hence, the output voltage of the signal charge sensor decreases.
Generally, solid-state imaging devices utilizing CCDs are provided with only one signal charge sensor that forms the output part thereof. Therefore, when it is desired to use the aforementioned solidstate imaging device in a darker place than usual, the external circuit provided external to the solid-state imaging device for amplifying the output signal of the signal charge sensor has to be switched over to another external circuit having a higher gain. Thus, the solid-state imaging device having only one signal charge sensor has the disadvantage in that the cost and size of the external circuit increase.
To overcome the above disadvantage, it has been known to provide a solid-state imaging device as shown in FIG. 2. The solid-state imaging device shown comprises vertical CCDs 21 arranged in rows and a horizontal CCD 22 connected to the vertical CCDs 21. To the final stage of the horizontal CCD 22 are connected three signal charge sensors 23, 25, and 26, arranged parallel to each other and having different sensitivities, dynamic ranges, etc. With this arrangement, one of the three signal charge sensors 23, 25, and 26 is selected that can provide the most suitable characteristics according to the conditions of use of the solid-state imaging device. As a result, there is no need to switch over the external circuit according to the conditions of use of the solid-state imaging device, which makes it possible to hold down the cost and reduce the size of the external circuit.
However, in the above prior art solid-state imaging device, since the final stage of the horizontal CCD 22 is connected to the three signal charge sensors 23, 25, and 26, arranged parallel to each other, the result is that the combined capacitances of the signal charge sensors 23, 25, and 26 are connected to the final stage of the horizontal CCD 22. This means that a greater capacitance is connected to the final stage of the horizontal CCD 22 than when only one signal charge sensor is connected. The resulting problem is that the output voltage that each of the signal charge sensors 23, 25, and 26 produces in response to the signal charge from the horizontal CCD 22 drops because of the large capacitance, which can render the solid state imaging device unserviceable.